Technical Field
The present invention generally relates to the field of the communication over an optical fiber. More in particular, the present invention concerns a communication system over a multi-mode optical fiber by means of the selection of a suitable sub-set of modes.
Description of the Related Art
An optical fiber is capable to provide a high bandwidth and thus it allows the transmission of digital information at high bit rates.
The wavelength division multiplexing technology (known as WDM) has allowed to increase the transmission capacity over a mono-mode optical fiber, but this could be not enough to satisfy the considerable increase of the required bandwidth.
An optical fiber with circular symmetry is capable of supporting a plurality of propagation modes and thus is referred as multi-mode optical fiber. The propagation modes are obtained by means of the solution of the Maxwell equations in the optical fiber, taking into account the boundary conditions between the core and the cladding of the optical fiber; a single propagation constant β is associated to each propagation mode, so that the electro-magnetic field variation along the axis z of the optical fiber is described only by one phase term of the type exp(i*β*z), wherein i is the imaginary unit. The vector modes are commonly indicated by TE (transverse electric), TM (transverse magnetic), EH (hybrid with electric dominance), HE (hybrid with magnetic dominance) and are classified based on two indexes v≧0, m≧1 having integer values, wherein index v indicates the angular trend of the electric field and index m indicates the radial trend of the electric field. Moreover, it is known that the trend of the transverse component of the electric (and magnetic) field of HE type (and of EH type) can have an even-type symmetry or an odd-type symmetry (see for example FIG. 2(b) of A. W. Snyder, W. R. Young, Modes of optical waveguides, Optical Society of America, 1978).
Techniques for transmitting information over a multi-mode optical fiber are known, such as for example the MIMO technique (Multiple Input Multiple Output): see H. R. Stuart, “Dispersive multiplexing in multimode optical fiber”, Science, 289, 281 (2000). This technique has the disadvantage to require at the reception side a complex electronic processing of the digital signal, thus requiring high performance processors, which consume high power values. Moreover, the use of a complex electronic processing has the disadvantage of increasing the latency, which could be unacceptable in some applications, such as for example in metropolitan telecommunications networks.
Therefore it would be a technical benefit to be able to find techniques which allow the management of the different modes as independent communication channels. It would be beneficial to avoid or reduce the mixing of the different modes with each other during the propagation and then be able to more easily discriminate such modes in the reception side, namely to prevent the problem known as “mode coupling”.